001    /*
002     * Copyright (C) 2007 The Guava Authors
003     *
004     * Licensed under the Apache License, Version 2.0 (the "License");
005     * you may not use this file except in compliance with the License.
006     * You may obtain a copy of the License at
007     *
008     * http://www.apache.org/licenses/LICENSE-2.0
009     *
010     * Unless required by applicable law or agreed to in writing, software
011     * distributed under the License is distributed on an "AS IS" BASIS,
012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013     * See the License for the specific language governing permissions and
014     * limitations under the License.
015     */
016    
017    package com.google.common.collect;
018    
019    import com.google.common.annotations.Beta;
020    import com.google.common.annotations.GwtCompatible;
021    
022    import java.util.NoSuchElementException;
023    import java.util.Queue;
024    
025    /**
026     * A queue which forwards all its method calls to another queue. Subclasses
027     * should override one or more methods to modify the behavior of the backing
028     * queue as desired per the <a
029     * href="http://en.wikipedia.org/wiki/Decorator_pattern">decorator pattern</a>.
030     *
031     * <p><b>Warning:</b> The methods of {@code ForwardingQueue} forward
032     * <b>indiscriminately</b> to the methods of the delegate. For example,
033     * overriding {@link #add} alone <b>will not</b> change the behavior of {@link
034     * #offer} which can lead to unexpected behavior. In this case, you should
035     * override {@code offer} as well, either providing your own implementation, or
036     * delegating to the provided {@code standardOffer} method.
037     *
038     * <p>The {@code standard} methods are not guaranteed to be thread-safe, even
039     * when all of the methods that they depend on are thread-safe.
040     *
041     * @author Mike Bostock
042     * @author Louis Wasserman
043     * @since 2 (imported from Google Collections Library)
044     */
045    @GwtCompatible
046    public abstract class ForwardingQueue<E> extends ForwardingCollection<E>
047        implements Queue<E> {
048    
049      /** Constructor for use by subclasses. */
050      protected ForwardingQueue() {}
051    
052      @Override protected abstract Queue<E> delegate();
053    
054      @Override
055      public boolean offer(E o) {
056        return delegate().offer(o);
057      }
058    
059      @Override
060      public E poll() {
061        return delegate().poll();
062      }
063    
064      @Override
065      public E remove() {
066        return delegate().remove();
067      }
068    
069      @Override
070      public E peek() {
071        return delegate().peek();
072      }
073    
074      @Override
075      public E element() {
076        return delegate().element();
077      }
078    
079      /**
080       * A sensible definition of {@link #offer} in terms of {@link #add}. If you
081       * override {@link #add}, you may wish to override {@link #offer} to forward
082       * to this implementation.
083       * 
084       * @since 7
085       */
086      @Beta protected boolean standardOffer(E e) {
087        try {
088          return add(e);
089        } catch (IllegalStateException caught) {
090          return false;
091        }
092      }
093    
094      /**
095       * A sensible definition of {@link #peek} in terms of {@link #element}. If you
096       * override {@link #element}, you may wish to override {@link #peek} to
097       * forward to this implementation.
098       * 
099       * @since 7
100       */
101      @Beta protected E standardPeek() {
102        try {
103          return element();
104        } catch (NoSuchElementException caught) {
105          return null;
106        }
107      }
108    
109      /**
110       * A sensible definition of {@link #poll} in terms of {@link #remove}. If you
111       * override {@link #remove}, you may wish to override {@link #poll} to forward
112       * to this implementation.
113       * 
114       * @since 7
115       */
116      @Beta protected E standardPoll() {
117        try {
118          return remove();
119        } catch (NoSuchElementException caught) {
120          return null;
121        }
122      }
123    }